In recent years, the domestic optical fiber gyro developed quickly. The engineering optical fiber gyro with medium and low precision was realized basically and was used successfully in many fields; the research and manufacture of the high-precision optical fiber gyro also went ahead like a raging fire. But generally speaking, there is a great gap between the domestic and foreign level. According to the basic working principle of optical fiber gyro:
                              Δφ          R                =                                            2              ⁢              π              ⁢                                                          ⁢              LD                                      λ              ⁢                                                          ⁢              C                                ⁢          Ω                                    (        1        )            
In which, ΔφR is the phase difference between two beams of light which propagates in opposite directions in optical fiber coil, L is length of the optical fiber coil, D is the average diameter of the optical fiber coil, λ is the wave length of light wave, C is the propagation speed of light wave in vacuum, Ω is the angle speed to which the optical fiber gyro is sensible. The phase difference ΔφR between two beams of light which propagates in opposite directions in optical fiber coil is proportional to the product of the length of the optical fiber coil and the average diameter LD. So the high-precision optical fiber gyro not only enlarges the diameter of the framework of the optical fiber coil, but also uses a longer optical fiber to wind the optical fiber coil. While, nonreciprocal phase difference between the two beams of light that propagates in opposite directions in the optical fiber coil not caused by rotation is one of the main error sources of the optical fiber gyro. The longer the optical fiber is, the higher the probability of occurrence of this kind of nonreciprocal phase difference is. Furthermore, this kind of nonreciprocal phase difference can be caused under the temperature environment by many factors such as the heat expansion of the framework of the optical fiber coil and unevenness of the heat field, and this problem of nonreciprocal phase difference must be solved by improving techniques. In the high-precision optical fiber gyro, how to reduce the influence of the framework to the optical fiber coil in high and low temperature, how to make the optical fiber coil reach the heat balance quickly and how to reduce the influence of outer environment to the optical fiber coil are all very important. One of the main reasons why the high-precision optical fiber gyro in our country has not achieved breakthrough progress is that the technical problems in optical fiber coil have not been solved completely.
In order to solve the technical problems of the optical fiber coil used in the high-precision optical fiber gyro, the R&D companies mostly use quadrupole symmetry winding method to wind optical fiber coil, and then perform curing to the optical fiber coil. The purpose of performing curing to the optical fiber coil is to improve the vibration characteristics of the coil and the property of repetition of the multiple electrification of the optical fiber gyro. The purpose of quadrupole symmetry winding method is to cause the two pieces of optical fibers of the optical fiber coil which are symmetrical relative to the middle point to undergo the same temperature field, so as to reduce the nonreciprocal phase difference caused by temperature. The temperature field inside the optical fiber coil, which influences the physical parameters such as refractive index and line expansion coefficient of the optical fiber, is related to the space location and time of the optical fiber. We take the refractive index caused by temperature as an example:
Considering the optical fiber coil with length of L as shown in FIG. 1 which is the diagrammatic drawing of temperature or stress disturbance in the optical fiber coil, there is a temperature disturbance on the small piece of optical fiber δz from which there is a distance of z to one of the ends of the optical fiber (M in this figure stands for the middle point of the length). Then the nonreciprocal phase difference introduced by this disturbance can be expressed as:
                                          δϕ            T                    ⁡                      (            z            )                          =                                            2              ⁢              π                        λ                    ⁢                                    ⅆ              n                                      ⅆ              t                                ⁢                      (            z            )                    ⁢                                    n              ⁡                              (                                  L                  -                                      2                    ⁢                    z                                                  )                                      C                    ⁢          δ          ⁢                                          ⁢          z                                    (        2        )                                                                    ⅆ              n                                      ⅆ              t                                ⁢                      (            z            )                          =                                                            ⅆ                n                                            ⅆ                T                                      ·                                          ⅆ                T                                            ⅆ                t                                              ⁢                      (            z            )                                              (        3        )            
In this equation, λ is the wave length of light wave, do/dT is the change rate of the refractive index of optical fiber to temperature, dT/dt is the time change rate of temperature at the place of δz, n is the refractive index of optical fiber, C is the light speed in vacuum. It can be seen from the equation (2) that the farther the micro-element disturbed by temperature is from the middle point of optical fiber, the larger the introduced nonreciprocal phase difference is. By integrating in the full-length range of the coil, the total phase difference introduced by temperature disturbance can be obtained:
                              Δϕ          T                =                                                            2                ⁢                π                ⁢                                                                  ⁢                n                                            λ                ⁢                                                                  ⁢                C                                      ·                                          ⅆ                n                                            ⅆ                T                                              ⁢                                    ∫              0              L                        ⁢                                                            ⅆ                  T                                                  ⅆ                  t                                            ⁢                              (                z                )                            ⁢                              (                                  l                  -                                      2                    ⁢                    z                                                  )                            ⁢                              ⅆ                z                                                                        (        4        )            
If the two pieces of optical fibers that are symmetrical relative to the middle point of the coil undergo the same temperature change, the integral in the equation (4) is zero and the phase difference introduced by temperature disturbance is also zero. The principle of refractive index change caused by stress is the same as the principle of that caused by temperature. If the two pieces of optical fiber that are symmetrical relative to the middle point of the coil undergo the same stress change, the phase difference introduced by stress disturbance is also zero.
The main problem using the above solution is: in the practical situation, because of non-ideal winding (e.g., not complete symmetry of winding, optical fiber cross phenomenon of each layer in the optical fiber coil) and faultiness of curing technique (uneven thickness of the adhesive, long heat balance time of the optical fiber, non-linearity of the temperature grads and so on), the two pieces of optical fibers that are symmetrical relative to the middle point of the coil cannot undergo the complete same temperature field; because the impregnation of adhesive in curing technique makes the optical fiber in the optical fiber coil receive force unevenly and the thermal expansion and cold contraction of the framework apply stress on the optical fiber, and etc., the optical fiber coil also cannot undergo the complete same stress field; in the temperature and mechanics vibration environment, the optical fiber coil cannot satisfy the precision requirement of the high-precision optical fiber gyro yet, so the design and technique must be further improved based on the existing base to advance the performance of optical fiber coil, so as to improve the precision of optical fiber gyro.
At present, the abroad patents and articles about winding method of optical fiber coil all only described a quadrupole symmetry winding method, and did not disclose the methods of producing non-framework optical fiber coils.
Concerning reports about framework, in domestic patent with publication number CN 101275835A and invention title of “Des-backbone winding ring clamp for non-upper edge optic fiber ring of optic fiber gyro”, the flanges on two sides of the framework are removed by means of clamp so that the upper part of optical fiber coil has no upper edge restriction and is in free state. But the implementation way by using clamp in that application is more complex, and the wheel hub and coil in framework are not separated, so the radial stress to the optical fiber coil produced by wheel hub can not be eliminated.